1. Field of the Invention
This invention relates to a distributed feedback semiconductor laser device that attains laser oscillation in a single longitudinal mode.
2. Description of the Prior Art
In a large-capacity light transfer system that has single-mode optical fibers and uses light with a long wavelength (e.g., 1.5-1.6 .mu.m), laser devices oscillating in a single mode even at the time of highspeed direct modulation can be used as a light source. These kinds of laser devices are called dynamic singlemode lasers, examples of which are distributed Bragg reflection laser devices (DBR laser devices) and distributed feedback laser devices (DFB laser devices).
In general, in Fabry-Perot resonance-type lasers, there are a number of longitudinal modes required to satisfy the phase conditions, and moreover there is no optical loss difference between the adjacent longitudinal modes, so that even though single-mode operation is achieved for a time, when there is high-speed direct modulation, the gain distribution changes remarkably, causing multi-mode operation and/or mode-hopping. When multi-mode operation takes place, beams with a number of wavelengths with different speeds of propagation are transmitted at the same time through the optical fibers, so that the resolution power of the signal decreases. When mode-hopping takes place, mode distribution noise occurs, resulting in the limitation of the transmission zone.
In order to solve the above-mentioned problems, DBR laser devices and DFB laser devices that can oscillate lasers in a single longitudinal mode have been developed.
FIG. 2 shows the structure in the vicinity of the resonator of a conventional DBR laser device, in which an optical waveguide 5 is disposed on both sides of the active region 4. The optical waveguide 5 has a diffraction grating, on its surface, with a periodically corrugated pattern by which the refractive index of the optical waveguide changes periodically. On the active region 4, there is provided an electrical current supply region. The DBR laser device with the above-mentioned structure oscillates laser light with an oscillation wavelength that is defined by both the periodicity of the diffraction grating formed on the optical waveguide 5 and the effective refractive index of the optical waveguide 5.
FIG. 3 shows the structure in the vicinity of the resonator region of a conventional DFB laser device, in which an optical waveguide 5 is disposed over the active region 4. The optical waveguide 5 has a diffraction grating, on its surface, with a periodically corrugated pattern on its surface by which the refractive index of the optical waveguide 5 changes periodically. This DFB laser device also oscillates laser light with an oscillation wavelength that is defined by both the periodicity of the diffraction grating and the effective refractive index of the optical waveguide 5. Since both the DBR laser device and the DFB laser device here, as shown in FIGS. 2 and 3, a corrugated structure (i.e., an optical waveguide with the diffraction grating), optical loss depends upon the wavelength. Thus, these laser devices can attain stabilized operating characteristics at a fixed wavelength that is determined by the size of the periodicity of the diffraction grating (diffraction intervals) and the effective refractive index of the optical waveguide.
However, the conventional DFB laser device with a built-in diffraction grating by which a longitudinal mode is regulated actually operates in two modes. That is, there is no operation mode when the lattice of the diffraction grating is located in such a manner that the phase difference that arises between the adjacent diffraction intervals of the diffraction grating is .pi./2, and when the lattice thereof is located in such a manner that the phase difference is in the vicinity of .+-..pi., there are operation modes with two equivalent gain threshold values.
In actual oscillation, the laser device oscillates in one or the other mode. However, it is difficult to obtain good reproducibility of oscillation in a single mode, because there is scattering of all of the parameters (for example, slipping of the corrugated pattern of the diffraction grating, and slipping of the layer thickness distribution), etc., in the production process for the laser device.
In order to control laser oscillation in the two longitudinal modes of the above-mentioned DFB laser device, a device structure such as that of FIG. 4 has been proposed in which there is a difference in phase of .pi./2 (a phase shift corresponding to 1/4 of the wavelength) between the right-sided and the left-sided diffraction gratings in the center portion of the optical waveguide 5, so that the laser oscillation wavelength becomes equal to the Bragg wavelength. Another device structure such as that of FIG. 5 has been proposed in which the distribution of the equivalent refractive index Neq that is changed symmetrically in relation to the center of the axis in the axis direction of the resonator in the center area of the optical waveguide 5 over which a diffraction grating is formed, can be, for example, formed by changes in the composition of the optical waveguide, so that only one of the operation modes can be selected. Of these approaches, the laser structure having a phase-shifted diffraction grating makes it possible theoretically to oscillate at the Bragg wavelength by a resonator that is formed by the diffraction grating. However, the method, in which a photoresistive film is formed to achieve the corrugated pattern of a diffraction grating, and based on the exposure of the interference pattern of laser light onto the said photoresistive film, the phase shift is directly added to this interference pattern, has not yet actually been achieved. This is because it is technically very difficult to achieve a 1/4 wavelength phase shift with regard to a diffraction grating having the pattern that changes periodically (i.e., peak.fwdarw.valley.fwdarw.valley.fwdarw.peak.fwdarw.valley) so as to obtain a diffraction grating having the pattern that changes periodically (e.g., peak.fwdarw.peak.fwdarw.valley.fwdarw.peak) due to the very small periodicity of the diffraction and the very narrow width of the peak portions. A method that is now known involves the use of two resists, positive and negative, and the formation of the laser light interference pattern on both the positive and the negative resistive films, by which the phases of these exposure regions are reversed, so that a phase shift of 1/4 of the wavelength can take place. This method, like that mentioned above, is technically difficult for the same reasons that have been described. Moreover, the distribution of the equivalent refractive index Neq must be symmetrically formed, which makes the production method of the laser device complicated.
Another method, in which the thickness of the optical waveguide is altered to change the effective refractive index, thereby attaining a shift of the Bragg wavelength in the resonator, so that one of the two longitudinal modes can be selectively stressed, has been proposed. However, this method is also technically difficult for the reasons that since the thickness of the crystal layers is changed so as to change the effective refractive index and a diffraction grating is formed on the top of the crystal layers, the surface of the underlying layer on which the diffraction grating is formed is not flat, which makes difficult the formation of an accurate pattern for the diffraction grating.